The present invention relates to a method of cleaving a brittle material using a point heat source which is capable of providing a thermal stress to the brittle material.
A conventional method for cleaving brittle materials such as semiconductor wafer, ceramics and glass by use of a heat source is disclosed in Japanese patent publication No. 3-13040. This conventional cleaving method will be described in detail with reference to FIGS. 1, 2 and 3. The brittle material is intended to be cleaved along a cleaving line 3. A recess 2 has been formed at a cleaving starting point which is defined as a crossing point of the cleaving line 3 and a side edge of the brittle material. A heat source 4 applies a heat locally at a point which is positioned on the cleaving line and in the vicinity of the recess 2 so that a tensile stress is generated in a direction along tangential lines of virtual isothermal lines 5. For this reason, the tensile stress causes a crack 6. The crack 6 propagates from a tip of the recess 2 toward the point of the heat source 4. As illustrated in FIG. 3, temperature distribution lines indicate that the temperature of the brittle material has a peak on the point of the heat source 4. Stress distribution lines indicate that a compressive stress appears on the point of the heat source 4, whilst the tensile stress appears around the point of the heat source 4. For this reason, a tip of the crack 6 propagates from the recess 2 toward the point of the heat source 4 so that the crack 6 extends to a position which is close to but distanced from the point of the heat source 4. The crack 6, however, does not reach the point of the heat source 4 because no tensile stress appears on the point of the heat source 4. As illustrated in FIG. 2, the point of the heat source 4 is moved along the cleaving line 3 so that the stress distribution lines move along the movement of the point of the heat source 4. As a result, the compressive stress and the tensile stress move along the movement of the point of the heat source 4. For this reason, the tip of the crack 6 further propagates along the cleaving line 3 toward the moved point of the heat source 4. The point of the heat source 4 moves from A, B to C sequentially, so that the tip of the crack 6 moves from P, P1 to P2. It can be understood that the locus of the movement of the point of the heat source 4 is different from the cleaving line 6 because the tip of the crack 6 propagates toward the point of the heat source 4.
In the above conventional cleaving method, the point of the heat source 4 is found by trial-and-error method, wherein a distance between the tip of the crack 6 and the point of the heat source 4 as well as a heating time are varied to determine an optimum heating point and an optimum heating time for effectively and efficiently adding the tensile stress to crack of the brittle material strip. The optimum heating point and the optimum heating time depend upon the material of the brittle strip and the width thereof. This means that the optimum distance between the tip of the crack 6 and the point of the heat source 4 as well as the optimum heating time depend upon the material of the brittle strip and the width thereof, for which reason the optimum distance between the tip of the crack 6 and the point of the heat source 4 as well as the optimum heating time are required to be found for every different material of the brittle strip and the different widths thereof. Even if the distance between the tip of the crack 6 and the point of the heat source 4 as well as the heating time are determined by the trial-and-error method, then those distance and heating time might be slightly different from the actual optimum distance and heating time. The conventional cleaving work is time-consuming procedure. This makes it difficult to realize automation of the laser beam machining.
In the above circumstances, it had been required to develop a novel method of cleaving a strip of brittle material free from the above problems.